Insulated electrical cable



June 7, 1938. J. w. GREENLEAF 2,120,095

INSULATED ELECTRICAL CABLE Filed 0013. 24, 1955 @lied/.dedos FeZzfed seslas INVENTR JOHN W. GREENLEAF ATTORNEYS Patented June 7, 1938 UNITED STATESkk PATENT OFFICE Rockbestos Products tion of Massachusetts Application October 24,

23 Claims.

This invention relates to insulated electrical conductors and has for its object the provision .of an improved article of this character. More particularly, the invention contemplates the provision of an insulated electrical conductor of improved electrical characteristics, highly resistant to water, substantially non-inflammable and capable of being sharply bent without being fractured or otherwise impairing the insulation.

The use of rubber insulated conductors for general distribution of electrical power has been customary for many years due to their comparatively high dielectric strength, resistance to moisture and low cost. During this period it has been well recognized that rubber insulated wires have certainundesirable characteristics, such as iniiammability, susceptibility to injury by rats and mice, and progressive deterioration due to age. Efforts have been made to overcome these faults by treating the outer braid covering with ame resisting compounds, by the application of armor coverings, and by the development of rubber compounds containing anti-oxidants to improve their ageing characteristics. While these eX- pedients have been to a degree successful and have increased the efficiency of such wires, the fact still remains that they are not actually reproof and when ignited by external heat or by arcing produced through overloads, accidental short circuits, or by loss of electrical insulating value through natural deterioration by age or injury by'rodents, they contribute to spread a fire and sometimes are the sole cause of serious losses.

Another form of insulated wire, which has been used to some extent, is that insulated with several layers of varnished cambric tape and finished with a braid of textile material which is treated with appropriate compounds to diminish its inflammability. This type of'insulated wire has the advantage of higher permissible operating temperature and possibly slower deterioration with age, but the insulating covering is inflammable and owing to the laminated structure of the insulating walls, it is not waterproof and consequently is not approved for installation Where moisture is apt to be present.

Protection of rubber and varnished cambric insulations from heat, rodents and oxidation has been accomplished by the application of densely felted asbestos layers over and under the high dielectric layer and by the application of woven braids of asbestos as the outer finish of wires and cables so insulated. Suitable neutral insulating compounds are used to impregnate the Corporation, a corpora- 1935, Serial No. 46,483

felted asbestos walls and the outer asbestos braid of such wires so that the high dielectric layers are sealed from contact with air, thus minimizing oxidation and deterioration, and also rendering the completed wire practically non-infiammable. Such wires, however, are of larger diameter and higher cost than the customary rubber and varnished cambric insulated wires, and therefore have not come into general use for domestic and industrial wiring.

I have discovered that rubber hydrochloride (natural rubber treated with hydrogen chloride) in conjunction with asbestos provides a remarkably satisfactory and eiective insulation for electrical conductors. Rubber hydrochloride possesses high dielectric strength and low ilammability, is highly resistant to water, oils, moderately concentrated acids and alkalies and most organic solvents. When produced in the' form of a thin lm, say about 0.001 inch in thickness, rubber hydrochloride is transparent, flexible and extensible to a slight degree, and in this form may be advantageouslyT employed for insulating electrical conductors.

Based on the foregoing discovery, the insulated electrical conductor of the present invention comprises a metallic conductor insulated by a plurality of surrounding layers of insulating media one of which is a relatively thin layer of rubber hydrochloride, and another is a relatively thick layer of asbestos. In practicing the invention, I prefer to use the rubber hydrochloride in the form of a film tape about 0.001 inch in thickness, and about 0.5-0.75 inch wide. Rubber hydrochloride is thermo-plastic to such an extent that adjacent layers of lm tape thereof can be sealed together by subjecting them to temperatures of from 105 to 130 C. for a short period of time. This thermo-plastic property of rubber hydrochloride permits its application to electrical conductors in lm tape form, wound helically with overlapping edges (as is customary with varnished cambric tape) and subsequently sealed into a substantially homogeneous and continuous tubular insulating layer which is water-proof, has high dielectric strength,. and has suicient elasticity and strength to retain its characteristics under all usual conditions of bending in service.

Dielectric tests of rubber hydrochloride in the form of a film 0.001 inch thick show a breakdown by puncture of 400G-5000 volts. This is extremely high for such a thin layer, and consequently it is possible to obtain the required dielectric strength for wire insulation with a much thinner 2 araches wall than when the usual rubber compounds or varnished cambric tapes are used. For example,

.six layers o 0.001 inch rubber hydrochloride film applied with overlapping edges to a conductor and subsequently sealed by the application oi heat have an equivalent dielectric breakdown of a Wall oi vulcanized crude rubber 0.045 inch thick.

Rubber hydrochloride is somewhat infiammable, butin association with asbestos, in accordance with the invention, this property is rendered substantially negligible for all practical purposes. The insulating layer of rubber hydrochloride may be in direct contact With the metallic conductor, or it may be interposed between two layers of asbestos. in all cases the layer o rubber hydrochloride is surrounded 'by a layer oi asbestos. Preferably the asbestos layer or layers are impregnated with a neutral flameproo insulating compound, and directly ralter impregnation subjected to a compressing and smoothing operation.

In the accompanying drawing Fig. l illustrates an insulated electrical conductor of the invention in which the rubber hydrochloride is in direct contact with the metallic conductor;

Fig. 2 illustrates an insulated conductor of the invention in `which the rubber hydrochloride is intermediate tvvo layers of asbestos; and

Fig. 3 illustrates a multiple conductor cable embodying the invention.

In manufacturing insulated conductors of the invention, particularly as illustrated in l, a film tape of rubber hydrochloride is spirally or helically wrapped around the metallic conductor in a suiclent number or" layers to provide the `required dielectric strength. En general, the required dielectric strength `will be provided by a Wall of rubber hydrochloride less than 0.015 inch in thickness. A layer o loosely ielted dry asbestos, or suitable thickness to protect the rubber hydrochloride from air, heat and mechanical injury, is then applied around. the spirally Wound layers of film tape. |The so-covered conductor is then passed through a neutral flameproof insulating compound maintained in liquid form at a temperature oi ll-l50 C. to saturate the asbestos fibers with the compound and to seal the laminated layers of film tape into a substantially continuous tube of Waterproof insulation. As the conductor emerges from the impregnatlng compound, it is' squeezed by revolving springs, so as to squeeze and' compact the asbestos layer and produce a smooth tough outer surface. An additional outer covering, for example, asbestos braid, may be applied for further mechanical protection, but will not be necessary except where unusually severe service is to be encountered.

In manufacturing insulated conductors of the invention, as illustrated in Fig. 2, the metallic conductor ls first covered with a felted Wall of asbestos fibers, applied in roving form, say to a Wall thickness of about 0.030 inch. It is my preierredn practice to lmpregnate this felted wall of asbestos with a flameprooflng compound. To this end the conductor -With its coveringof felted asbestos is passed through a neutral flameproof insulating compound maintained in liquid form at a temperature of 110-150" C., allowing suillcient time for the liquid compound to thoroughming out the excess or the impregnated compound.

The conductor and its 'wall of impregnated asbestos is next covered with a thin wall of rubber hydrochloride. 'll'o this end, a film tape of rubber hydrochloride is spirally or helically Wrapped around the impregnated asbestos in a` suiiclent number of layers to provide the required dielectric strength. In general, the required dielectric strength will be provided by a wall of rubber hydrochloride less than 0.015 inch in thickness. .A layer of loosely elted dry asbestos of suitable vthickness to protect the rubber hydrochlou ride irom air, heat land mechanical injury, is then applied around the spirally wound layers oi illm tape. The so-covered conductor is then passed through the neutral rlameproof insulating compound maintained in liquid form at a temperature of 110-150" C. to saturate the last applied layer of asbestos ibers with the compound and to seal the laminated layers oi nlm tape into a substantially continuous tube of waterproof insulation. .i-ls the conductor emerges from the lmpregnating compound it is squeezed by revolving springs, so as to squeeze and compact the asbestos layer and produce a smooth tough outer surface. i-ln additional outer covering, for example, asbestos braid, may be applied ior iurther mechanical protection, but will not be necessary except Where unusually severe service is to be encountered.

The laminated layers of nlm tape, due to the thermo-plastic property or" rubber hydrochloride, are subjected to sufficient heat during immersion in the impregnating compound to merge the overlapping layers of film tape into a substantially homogeneous and continuous tube of rubber hydrochloride. The time required to thus seal andmerge the layers of illm tape depends on the number of layers and the temperature. When the temperature of the impregnating compound ls from 1l0-l50 C., the time required to seal and merge the layers of film tape (0.001 inch thick) is approximately one minute per layer. Ordinarily, not more than ten layers of iilm tape 0.001 inch thick Will be required to impart adequate dielectric strength to the nlshed conductor. It will of course be understood that the layer of impregnated and compacted felted asbestos Will be considerably thicker than the layer of rubber hydrochloride.

lirr the multiple conductor cable of Fig. 3, the individual conductors are preferably formed of stranded wire, and are initially covered yvyvith insulation of the character illustrated in either Fig. l or Fig. 2. The insulated conductors' (two or more) are then twisted and covered with felted asbestos to provide a substantially cylindrical structure. A nlm tape of rubber hydrochloride is spirally or helically wrapped around the asbestos-covered cable in a sulcient number of layers to provide the required dielectric strength. In general, the required dielectric strength will be provided by a Wall of rubber hydrochloride less than 0.015 inch in thickness. A layer of loosely felted dry asbestos of suitable thickness to protect the rubber hydrochloride from air, heatl and mechanical injury, is then applied around the spirally wound layers of iilm tape. The socovered cable is then passed through a. neutral flameproof insulating compound maintained in liquid form at a. temperature of 110150 C. to saturate the asbestos libres with the compound and to seal the laminated layers of lm tape into a. substantially continuous tube of waterproof in- Cil yals

sulation, As the cable emerges from the impregnating compound, it is squeezed by revolvingy springs, so as to squeeze and compact the asbestos layer and produce a smooth tough surface. If desired, the common layer of asbestos iirst applied around the twisted conductors may be similarly impregnated with the fiameproong compound, compressed and smoothed, prior to the application of the layer of rubber hydrochloride. If desired, the cable may be covered wtih an outer protective layer of asbestos braid, or the like.

I have found that the application of heat to the layer of rubber hydrochloride not only serves to seal and merge the laminated layers of iilm tape together, but so softens the rubber hydrochloride that in the subsequent compressing operation the iibers of asbestos are pressed into the rubber hydrochloride, so that the asbestos bers are at least partially embedded in and tenaciously adhere to the rubber hydrochloride. 'I'he resulting association of the rubber hydrochloride and asbestos is so intimate that the two insulating media cannot be separately stripped from the conductor, and only by careful inspection can the layer of rubber hydrochloride be detected in the insulation when stripped from the conductor. This intimate association of rubber hydrochloride and asbestos effectively overcomes the inilammability of the rubber hydrochloride, and as a consequence the finished conductor is practically non-inflammable. l

The insulated conductor of the invention is o smaller diameter than insulated conductors of the prior art of equivalent dielectric strength, is substantially waterproof and ameproof, does not deteriorate with age, may be bent sharply without rupturing or otherwise impairing the insulation, and can be manufactured at relatively low cost.

1. An insulated electrical conductor comprising a metallic conductor insulated by a plurality of surrounding layers of insulating media one of which is a relatively thin layer of krubber hydrochloride in such intimate contact with an adjacent layer of asbestos that the contacting fibers of asbestos are at least partially embedded in and tenaciously adhere to the rubber hydrochloride.

2; An insulated electrical conductor comprising a metallic conductor insulated by a plurality of surrounding layers of insulating media including a relatively thin insulating layer of high dielectric lstrength composed principally oi rubber hydrochloride, and a relatively thick layer of asbestos.

3. An insulated electrical conductor comprising a metallic conductor surrounded by a sub- A stantlally homogeneous and continuous ilexible layer of rubber hydrochloride, and a layer of felted asbestos surrounding said layer of rubber hydrochloride. y

.4. An insulated electrical conductor comprising a metallic conductor insulated by a plurality of surroundinglayers of insulating media including a relatively thin layer of rubber hydrochloride, and a relatively thick layer of asbestos.

5. An insulated electrical conductor comprising a metallic conductor surrounded by a substantially homogeneous` layer of rubber hydrochloride less than 0.015 inch in thickness, and a relatively thicker layer of felted asbestos surrounding said layer of rubber hydrochloride.

6. An insulated electrical conductor comprising a metallic conductor surrounded by a substantially continuous andA homogeneous tube of high dielectric strength insulation Acomposed principally of rubber hydrochloride, and a layer of felted asbestos impregnated with a flameproofing compound surrounding said tube.

7. An insulated electrical conductor comprising a metallic conductor surrounded by a substantially continuous and homogeneous tube of high dielectric strength insulation composed principally of rubber hydrochloride, and a layer of asbestos impregnated with a ilameproong compound surrounding said tube and in such intimate contact therewith that the contacting iibers of asbestos are at least partially embedded in and tenaciously adhere to the rubber hydrochloride.

8. An insulated electrical conductor comprising a metallic conductor surrounded by and in direct contact with a substantially homogeneous and continuous ilexible layer or rubber hydrochloride, and a layer of felted asbestos surrounding said layer of rubber hydrochloride.

9. An insulated electrical conductor comprising a metallic conductor surrounded by and in direct contact with a substantially homogeneous and continuous layer of rubber hydrochloride, and a layer of asbestos surrounding said layer of rubber hydrochloride and in such intimate contact therewith that the contacting iibers of asbestos are at least partially embedded in and tenaciously adhere to the rubber hydrochloride. l

l0. An insulated electrical conductor comprising a metallic conductor surrounded by and in direct Contact with a substantially homogeneous layer of rubber hydrochloride less than 0.015 inch in thickness, and a relatively thicker layer of felted asbestos impregnated with a iiameprooiing compound surrounding said layer of rubber hydrochloride and in such intimate contact therewith that the contacting fibers of asbestos are at least partially embedded in and tenaciously adhere to the rubber hydrochloride.

l1. An insulated electrical conductor comprilsing a metallicconductor surrounded by and in direct contact with a substantially continuous and homogeneous layer of rubber hydrochloride, and a layer of asbestos impregnated with a ilameproofing compound surrounding said layer of rubber hydrochloride and in such intimate contact therewith that the contacting bers of asbestos are at least partially' embedded in and tenaciously adhere to the rubber hydrochloride.

l2. An insulated electrical conductor comprising a metallic conductor surrounded by at least two separate relatively thick layers of asbestos, and a relatively thin layer of rubber hydrochloride interposed between the two relatively thicker layers of asbestos.

13. An insulated electrical conductor comprising a metallic conductor surrounded by and in direct contact with a layer of asbestos, a substantially homogeneous layer of rubber hydrochloride surrounding said layer of asbestos, and a second layer of asbestos surrounding said layer of rub- Vber hydrochloride.

15. An insulated electrical conductor comprising a metallic conductor surrounded by and in direct contact with a layer of asbestos impregnated with a ameproong compound, a substantially continuous and homogeneous layer of rubber hydrochloride surrounding and in contact with said layer of impregnated asbestos, and a layer of asbestos impregnated with a Iiameproofing compound surrounding said layer of rubber hydrochloride and in -such intimate contact,

nated with a flameproong compound surrounding said tube and in such intimatecontact therewith that the contacting fibers of asbestos are at least partially embedded in and tenaciously adhere to the rubber hydrochloride.

17. An insulated electrical conductor comprising a metallic conductor insulated by a plurality of surrounding layers of insulating media includlng at least two relatively thicklayers of asbestos, and a substantially homogeneous layer of rubber hydrochloride less than 0.015 inch in thickness interposed between the two relatively thicker layers of asbestos.

18. A multiple conductor cable comprising a plurality of individually insulated metallic conductors surrounded by a substantially homogeneous and continuous layer of rubber hydrochloride, and a layer of asbestos surrounding said layer of rubber hydrochloride.

19. A multiple conductor cable comprising a plurality of insulated metallic conductors surrounded by a substantially homogeneous and continuous layer of rubber hydrochloride, and a layer 0f asbestos surrounding said layer of rubber hydrochloride, each of said insulated metallic conductors being individually insulated by a plurality of surrounding layers of insulating media one of which is a relatively thin layer of rubber hydrochloride and another is a relatively thicker layer of asbestos.

araches.

20. A multiple conductor cableA comprising a plurality of individually insulated metallic conductors surrounded by a plurality of layers of insulating media including at least two relatively thick layers of,asbestos, and a relatively thin layer of rubber hydrochloride interposed between the two relatively thicker layers of asbestos.

21. A multiple conductor cable comprising a y surrounding said layer of asbestos and in direct contact therewith, and a layer of asbestos impregnated with a iiameproong compound surrounding said tube and in such intimate contact therewith that the contacting bers of asbestos are at least partially embedded in and tenaciously adhere to the rubber hydrochloride.

22. A multiple conductor cable comprising a plurality of individually insulated metallic conductors surrounded by a plurality of common layers of insulating media including two relatively thick layers of asbestos, and a relatively thin layer of rubber hydrochloride interposed between the two relatively thicker layers of asbestos and in such intimate contact therewith that the contacting bers of asbestos are at least partially embedded in and tenaciously adhere to the rubber hydrochloride.

23. A multiple conductor cable comprising a plurality of metallic conductors each individually insulated by a plurality of surrounding layers of insulating media including a relatively thin layer of rubber hydrochloride` and a relatively thicker layer of asbestos, said individually insulated conductors being twisted into a cable and covered with`a' common layer of asbestos, a relatively thin layer of rubber hydrochloride surrounding said common layer of asbestos and in direct contact therewith, and a layer of asbestos impregnated with a flameproong compoundsurrounding said layer of rubber hydrochloride and in such intimate contact therewith that the contacting fibers of asbestos are at least partially embedded in and tenaciously adhere to the rubber hydrochloride.

A 'JOHN W. GREENLEAF.

resent No. .2,iao,o95.y

JOHNv W.' "GREENIEAF i June 7a I195811 Iii-is herebycertifed that error appears in the printed specifl ication of the above numbered patent requir ing correction as follows: Page 5, first column, line l0, for wafi-sib.",read with; and second column, line 20, claim 8, for or, read of;` and that the said Letters Patent should be read with this correction therein that the same may conformi tothe record of' the case in the Patent -officeo Signed and sealed this md. day o August, A. D.. 1958,.

(Seal) v Leslie Frazer,

Acting; Commissioner of Patents, 

